Currently communication systems typically employ linear modulation schemes that exhibit high peak-to-average power ratios to achieve high capacity. Signals subjected to such modulation have a wide dynamic range, infrequently achieve peak levels, and frequently operate below peak levels. To provide linear amplification for signals subjected to such modulation, traditional amplifier architectures operate significantly below their peak level. This generally results in poor efficiency.
A number of techniques exist to improve efficiency based on adjusting the amplifier supply voltage. Notable amongst the supply voltage-based efficiency enhancement schemes are those of envelope tracking and envelope elimination and restoration.
These two techniques improve efficiency by dynamically adjusting or tracking the supply to the amplifier device in harmony with the power demand or envelope of the signal to be amplified. When applied to an amplifier stage, the dynamic adjustment of the supply alters the bias of the amplifying element which results in high energy conversion efficiency between the amplifier supply and the amplified output signal.
A known problem in implementing dynamic supply adjustment amplifiers is the need for tight timing synchronisation or alignment between the dynamic supply voltage applied to the amplifying element and the energy demanded by the signal to be amplified. In other words, there is a need for the envelope of the power supply delivered to the amplifier to align with the envelope of the amplified output signal. The absence of such alignment may result in a reduction in the power efficiency of the power amplifier and distortion of the amplifier output signal, which distortion is unacceptable to the end application. These problems also present extra challenges for any spectral improvement systems, such as pre-distortion blocks, associated with the amplifier stage.
A particular advantageous application of envelope tracking amplifiers is in solving the problem of high efficiency radio frequency amplification of modulation signals having high peak-to-average ratio. However in such an application the amplification element, being a radio frequency (RF) transistor, tends to produce unacceptable output distortion in the presence of a timing misalignment of the dynamic supply and input signal to amplify.
Historically alignment has been carried out manually using either maximum power to establish crude alignment or symmetry and best adjacent channel power measurements with the use of a non-memory adaptive pre-distortion system.
A known prior art solution to the timing alignment problem is to monitor the amplifier output signal, with resulting timing related distortion, and then to adjust the timing of the dynamics or the timing of the signal to be amplified using an associated delay element in order to set an optimal timing for minimal distortion.
The implementation of timing tracking in this way lends itself to an amplifier system that also deploys adaptive linearity control in the form of a pre-distortion block. In this instance the circuitry necessary for accurate measurement and comparison of the amplifier output signal is already present within the amplifier linearity control system.
For amplifiers that do not require adaptive linearity control, or those that employ alternative linearity control mechanisms, the cost and complexity of providing output measurements and, in particular for RF systems, down-conversion frequency translation, is undesirable and may be prohibitive.
It is an aim of the invention to provide an improved technique to address one or more of the above-stated problems.
In particular it is an aim of the invention to provide a timing alignment mechanism that does not require direct measurement of the amplifier output signal and therefore does not require any associated measurement circuitry.